The present invention relates generally to electronic memory and, more particularly, to a ferroelectric memory device and a method of reading such a device.
Ferroelectric memory is a type of nonvolatile memory that utilizes the ferroelectric behavior of certain materials to retain data in a memory device in the form of positive and negative polarization, even in the absence of electric power. A ferroelectric material contains domains of similarly oriented electric dipoles that retain their orientation unless disturbed by some externally imposed electric force. The polarization of the material characterizes the extent to which these domains are aligned. The polarization can be reversed by the application of an electric field of sufficient strength and polarity.
FIG. 1 illustrates a prior art ferroelectric cell 10 in a ferroelectric memory array. A ferroelectric material 16 having a polarization P is sandwiched between a conductive word line 20 and a conductive bit line 22. An electric field may be applied to the ferroelectric cell by applying an electric potential (voltage) between the word line and the bit line so as to effect changes in the polarization of the ferroelectric material.
When a positive voltage of sufficiently large magnitude is applied to the cell, all of the domains in the cell are forced to align, to the extent possible, in the positive direction. If the voltage is then reduced to zero, some of the domains switch their orientation (also referred to as rotating, flipping or reversing), but most of the domains retain their orientation. Thus, the ferroelectric material retains a remnant polarization in the positive direction.
If a negative voltage of sufficiently large magnitude is then applied to the word line relative to bit line, all of the domains are forced to switch their orientation. Removing this negative voltage allows some of the domains to switch, but the cell polarization retains a remnant polarization in the negative direction until it is disturbed again.
For purposes of data storage, the ferroelectric cell 10 is considered to be in the logic xe2x80x9c0xe2x80x9d (zero) state when the polarization is positive, and the logic xe2x80x9c1xe2x80x9d (one) state when the polarization is negative.
A certain amount of charge is required to switch the polarity of a domain. This charge release provides the fundamental principle for a xe2x80x9cdestructivexe2x80x9d read of a ferroelectric cell. For example, the state of a cell can be read by observing the charge released from the cell while applying a positive voltage sufficient to switch the polarization of the cell. A large charge release indicates that the cell was a logic one, whereas little or no charge release indicates that the cell was a logic zero. The cell ends up in the zero state, regardless of its state before the read operation. Thus, a cell that was in the one state must then be rewritten as a one if further data retention is required.